gfs2: Rename bitmap.bi_{len => bytes}

This field indicates the size of the bitmap in bytes, similar to how the
bi_blocks field indicates the size of the bitmap in blocks.

In count_unlinked, replace an instance of bi_bytes * GFS2_NBBY by
bi_blocks.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Reviewed-by: Steven Whitehouse <swhiteho@redhat.com>